Wavelength division multiplexing (WDM) technology and the more recent dense wavelength division multiplexing technology (DWDM) have pushed manufacturers of optical transmission fibres to produce fibres having a very small optical attenuation across a broad wavelength region of the transmission spectrum. A particular problem that has been dealt with in the last decade is the attenuation due to the absorption by the optical fibre, during its manufacturing, of hydroxyl radicals (—OH) and/or H2O molecules present in the optically active region of the fibre. The —OH groups and water can be formed during manufacturing of optical fibre preforms when the latter are exposed to hydrogen, which can be available from various sources. Contamination from hydroxyl radicals or water causes a linear attenuation at 1380 nm, visible in the attenuation spectrum as a peak generally referred to as the “water peak”.
A common procedure to obtain a glass preform to be drawn down to an optical fibre of typical diameter of about 125 μm comprises a first process for producing a core rod of solid glass, also indicated as core cane, and a second process in which an overcladding is added to the core rod by a suitable process, e.g. by deposition of soot about the core rod or by jacketing the core rod in a glass tube (rod-in-tube—RIT—technology).
WO 01/47822 relates to a method of fabricating a cylindrical glass body for use in manufacturing an optical fibre with low water peak, the method comprising the steps of: forming a consolidated glass precursor body for use in manufacturing an optical fibre preform, said precursor body comprising at least a partial core region of said preform; exposing said glass precursor body to deuterium gas, and forming additional cladding on said precursor body to form an optical fibre preform. The precursor body is a glass core cane comprising a core region and at least a portion of the glass cladding region. Prior to additional overclad soot being added to the core cane, the core cane is suspended within a furnace and treated with deuterium for a time and temperature which is sufficient to prevent rewetting of the consolidated glass core cane.
The soot precursor body, especially when made by outside vapour deposition (OVD), vapour axial deposition (VAD), typically has a core region surrounded by a near clad region, also indicated as inner clad region, often made of pure silica. Optionally, the inner cladding region comprises one or more layers of doped silica. The resultant precursor body is then dried and consolidated to form a core glass body. A stretching often follows the consolidation in order to reduce the diameter of the glass body which is then severed into a plurality of core rods. Alternatively core rods may also be made by inside deposition processes such as Modified or Furnace Chemical Vapour Deposition (MCVD/FCVD), or Plasma Chemical Vapour Deposition (PCVD).
A parameter that is often used as indicative of the cost and performance of the produced optical fibre is the core-to-clad ratio of a glass core rod, a/b, where a is the radius of the core region of the core rod and b is the external radius of the inner clad region surrounding the core region. The larger the value of a/b, the higher can be the yield of the manufacturing process since more core rods can be obtained from a single soot preform.
US 2011/0023551 discloses a method of fabricating core rod sections useable for the production of finished optical fibre preforms. The method is said to allow an increase of the cladding to core ratio, indicated with D/d. The disclosed method comprises inserting two or more core body pieces end-to-end axially inside a glass cylinder, thereby defining joints between adjacent ones of the inserted core body pieces; mounting the glass cylinder with the contained core body pieces vertically in a region of a furnace; heating the glass cylinder and core body pieces together in the furnace, thereby elongating the cylinder and the core body pieces contained in the cylinder; collapsing the elongated cylinder so as to form a finished core rod; and cutting the finished core rod at one or more positions along the length of the core rod that coincide with the joints between adjacent ones of the core body pieces, thereby yielding a number of core rod sections one or more of which are useable for the production of optical fiber preforms. The core body pieces have a D/d of four or less, whereas the finished core rod is formed with a D/d ratio of more than five.
WO 2012/010212 describes a bend-resistant optical fibre having a core region and a cladding region surrounding the core region and comprising a void-containing annular layer doped with fluorine. The fluorine doping can be performed during the consolidation process following a soot deposition of a silica layer over the core blank of a preform.